The present invention relates to an optical data reader, and more particularly, to an optical data reader provided with a servo system which maintains an orthogonal relation between the recording surface of a disk forming a recording medium and the optical axis of an irradiating light beam directed onto the recording surface.
If there is a significant deviation from an orthogonal relation between the recording surface of a disc and the optical axis of the irradiating light beam, the shape of the pick-up spot converged on the recording tracks of the disc will be distorted, and therefore data from the adjacent tracks may "leak" into the output signal of the pickup, that is, cross-talk may occur. A major cause of such deviation is due to a basin-like distortion of the disc which occurs upon cooling of the disc following its molding.
Manufacturing standards for optical data recording discs include tolerance limits on the amount of the basin-like distortion. Typical values of such tolerance limits are, for example, -2.5 to +1.5 mm for discs having an outer diameter of 20 to 30 cm and +0.5 to -0.5 cm for discs having an outer diameter of about 12 cm.
Often optical data readers are provided with tilt servomechanisms for maintaining the orthogonal relationship between the optical axis of the irradiating light beam directed onto the recording surface of the disc and the surface of the disc in order to reduce the amount of cross talk. The present applicant has previously proposed such an optical data reader provided with a tilt servomechanism in Japanese patent application No. 70968/81 (published as Japanese patent Publication No. 186237/82) and as shown in FIG. 1.
With reference to FIG. 1, a recording disc 1 set on a turntable 3 rotated by a spindle motor 2 is pressed to the turntable 3 by a downwardly urged support member 4 and a clamper 5 rotatably supported by the latter. As can be understood from this figure, the disc 1 is deformed into a basin-like shape. On one side of the spindle motor 2 a slide base 6 is movably arranged in the radial direction of the disc 1. A case 7 is pivotally mounted on the slide base 6 through a support shaft 8. In the case 7 is disposed an optical system including an objective lens 9, which directs a laser beam onto the recording surface of the disc, and a detector, which senses the quantity of light reflected from the recording surface. The objective lens 9, which is used for converging the irradiating light beam, having an optical axis 10a, 10b, onto the disc recording surface, is arranged so as to be movable in both the optical axis direction (focusing direction) and the direction normal to the former, that is, in the tracking direction, with respect to the case 7, thereby to be driven in both directions.
Further, the shaft 8 which pivotally supports the case 7 is positioned on an extension of the optical axis 10a, 10b. A holding mechanism for holding the abovementioned optical system is composed of the slide member 6, the case 7, and peripheral small members related thereto.
FIG. 1 shows two conditions, in one of which the above-mentioned holding mechanism is in the position before reproducing, and in the other one of which it is in the reproducing position. A part 11 of the tilt servomechanism is arranged on the pivotable end side of the case 7 on the slide base 6. This tilt servomechanism (only a part of which is shown) includes a support shaft 8, a detector, including a light-emitting element and two light-detecting elements for detecting deviations in the orthogonal relationship between the optical axis 10a, 10b and the recording surface of the disc 1 and producing a detection signal in response thereto; a control section for normally and reversely rotating a drive motor in accordance with the detection signal; a rack member secured to the pivotable end part 7a of the case 7 and extending in the pivoting direction of the pivotable end 7a and formed therein with a rack section extending over substantially the entire length of the rack member; a speed reducing mechanism including a gear meshed with the rack section; and a drive motor for applying drive power to the above-mentione rack section to move the case 7 through the speed reducing mechanism. All of the above-mentioned components, except the detector which is mounted on the case 7, are mounted on the slide base 6.
The operation of the above-described prior art arrangement will now be explained.
If the holding mechanism composed of the slide base 6 and the case 7 moves from its prereproducing position, which the optical axis is illustrated by line 10a, by a predetermined distance and reaches a position below the inclined section of the recording disc 1, the optical axis seen as line 10b and the disc recording surface no longer will have an orthogonal relation therebetween. The above-mentioned detector detects this condition and delivers a detection signal to a control section. The control section rotates the above-mentioned drive motor normally or reversely to reestablish the orthogonal condition between the optical axis 10a, 10b and the disc recording surface.
However, although a nonorthogonal relationship can be corrected, the distance between the disc recording surface and the objective lens 9 changes from A to B, as indicated in FIG. 1, after this correction to the orthogonal relationship. Therefore, the objective lens 9 must be displaced parallel to the optical axis (corresponding to A-B) to converge the irradiating light beam onto the disc recording surface.
Moreover, as mentioned above, the prior art tilt servomechanism has a large number of component parts, and therefore is very expensive.